1. Field of the Invention
The present invention relates to a sealing structure.
2. Description of the Related Art
A conventional sealing structure will be described with reference to a vacuum apparatus. For example, as set forth in FIG. 5, a vacuum apparatus, such as a plasma enhanced CVD system for producing semiconductor devices, is assembled from a plurality of chamber members, e.g. a housing member 3, a cover member 2, a vacuum system member 4 and a flange 5, while sealing these chamber members with O-rings O1, O2 and O3.
The vacuum apparatus set forth in FIG. 5 has three sealing sections S1, S2 and S3. In detail, the housing member 3 and the cover member 2 are sealed with the O-ring O1 at the sealing section S1, the housing member 3 and the vacuum system member 4 are sealed with the O-ring 03 at the sealing section S3, and the cover member 2 and the flange 5 are sealed with the O-ring O2 at the sealing section S2. A vacuum apparatus may be assembled from more chamber members depending on the type and application of the vacuum apparatus, and the number of the sealing sections also varies with this. In FIG. 5, symbol W represents a substrate, symbol EL represents a lower electrode, symbol EU represents an upper electrode provided with a shower box, symbol MFC represents a mass flow controller, symbol M/B represent a matching box, symbol TMP represents a turbo molecular pump and symbol RF represents a high-frequency power source.
Stainless steel has generally been used for chamber members, and aluminum alloys have also been used for reducing weight of the apparatus and improving homogeneity of temperature in the chamber.
Since aluminum alloys have lower hardnesses compared to stainless steel, the O-ring seating face sinks or yields to form a dent when repeatedly used. In particular, such denting is noticeable in low hardness aluminum alloys such as JIS A5052. This denting causes an increased leakage rate of the vacuum apparatus.
An improvement in O-rings has been tried to prevent dent formation and decrease the leakage rate of the vacuum apparatus. The improved O-ring is a SUS304 O-ring (Hv: 200 to 300) coated with pure aluminum (Hv: 30). Although this O-ring somewhat increases the number of repeated cycles until a dent forms, it does not perfectly prevent denting.
Several methods for hardening the surface of the chamber member have been proposed as follows:
(1) The O-ring seating face is coated with a TiN film having a thickness of approximately several dozen .mu.m to increase the hardness of the O-ring seating face.
In this case, since the substrate still has a low hardness, a dent forms on the substrate and the coated film also yields accompanying the dent, resulting in leakage. When using a JIS A5052 material (Hv: 60 to 70), leakage is noticeable. The TiN film is coated at a high temperature of 200 to 250.degree. C. When using an aluminum alloy, of which the hardness is increased by age hardening, for example, JIS A2219-T87 (Hv: 130) as a substrate, the aluminum alloy is softened to a Hv value of 50 to 60 during film coating, and thus a dent forms as in JIS A5052 and thus leakage is inevitable.
(2) FIGS. 7A-7E show another conventional sealing structure. In this conventional sealing structure, a groove 108 is formed in the O-ring seating face 107 as shown in FIG. 7B. A Ni wire 109 is placed in the groove 108 as shown in FIG. 7C. The entire Ni wire 109 is melted and fused in the division member, i.e., a base material 106, by welding as shown in FIG. 7D to form a weld section 110, which is made of an Al--Ni alloy (Hv: 100 to 200). The surface is finished by mechanical working to smooth it as shown in FIG. 7E.
However, it has been found that the surface after finishing occasionally has coarse dimples in this case. It is considered that these dimples form due to the following reason. Oxygen which is present as oxide on the aluminum alloy surface evolves as oxygen bubbles during welding and is trapped in the alloy. The bubbles are released from the matrix during surface finishing but bubble marks still remain on the finished surface as dimples. Such dimples also work as a leakage source. Although bubble marks are not always present on the chamber member, this technology is unsuitable for vacuum apparatuses having many sealing sections, because the degree of vacuum decreases if one of a plurality of vacuum units has a leaking section.
Further, the hardness of the weld section 110 is not uniform and fine cracks due to nonuniform hardness are found. These cracks also work as a leakage source.